1. Field of the Invention
The present invention relates to a mobile communication terminal and a communication control method for shortening the time required for connection to a wireless system and for decreasing the consumption of electric power.
2. Description of the Related Art
A plurality of wireless systems have nowadays been placed in practice, such as CDMA 2000 (Code Division Multiple Access), EV-DO (Evolution-Data Optimized), W-CDMA (a European version of Wideband CDMA) and the like. In the future, it is expected that even the same wireless system will be operated at different frequency bands such as a 800 MHz band, a 1.7 GHz band, a 2 GHz band and so on. A mobile communication terminal has been developed that can be used for a plurality of wireless systems or over a plurality of frequency bands.
When a power source circuit is closed, the mobile communication terminal performs acquisition acquirement of control signals transmitted from the base stations of the systems (inclusive of when different frequency bands are used by the same system) in accordance with an order based on the data of a preferred roaming list stored in the terminal and is shifted into an idle state after the synchronization is synchronized. Even in the case of the control signal of the system in the idle state being lost or even in the case where it becomes difficult to maintain the idle state due to a deterioration in the electromagnetic wave environment, the mobile communication terminal carries out the acquisition operation in accordance with an order based on the data of the preferred roaming list and is shifted into the idle state with the system that is acquired.
There is a technology that has been known for varying the order of acquiring a plurality of systems depending upon the reception quality (RSSI or Ec/Io) in the idle state. For example, Japanese Laid-Open Patent Application No. JP-A-2003-153325 discloses an art where two systems A and B are registered in this order to the preferred roaming list. If this system A that had been in the idle state is lost, there is a high probability of acquiring system A if the reception quality of system A is better than a given condition. In addition if acquired, the acquisition operation is carried out again in order of A→B.
However, if the reception quality of system A is poorer than the given condition, the probability of acquiring system A again becomes low. In this case, acquisition of system A is not attempted but the operation is started for acquiring system B in order to shorten the time for re-acquisition and to decrease the consumption of electric power.
In the mobile communication terminal capable of using a plurality of wireless systems however, the system in the idle state may not often be the same as the system that originates. For example, the system can be placed in the idle state with CDMA 2000 and originate in a wireless LAN only at the time of receiving the streaming of dynamic images.
As a concrete example, there is contrived a mobile communication terminal that operates as shown in FIG. 10. It is presumed that the mobile communication terminal can be used with three systems A, B and C, and is placed in the idle state with system C. If the origination operation is performed while in the idle state with system C (step 102), the operation is conducted to acquire system A (step 104). Here, if system A is acquired, the communication starts with system A (step 106). If system A is not acquired, the operation works to acquire system B (step 108). Here, if system B is acquired, the communication starts with system B (step 110). If system B is not acquired, the operation works to acquire system C (step 112). Here, if system C is acquired, the communication starts with system C (step 114). If system C is not acquired, the origination fails and an idle state is assumed (step 116).
Here, concerning the mobile communication terminal that performs the above sequence, origination in an area which is out of the service area for system A results in the waste of time and electric power if it is attempted to acquire system A at step 104. In an area which is out of the service area for system A, the origination shall not be performed to system A but shall be performed to system B or to system C irrespective of the condition under which system C is receiving the electromagnetic waves.
However, the art disclosed in the above Japanese Laid-Open Patent Application No. JP-A-2003-153325 does not give consideration to changing the system at the source of origination which depend upon the area where the mobile communication terminal is placed in the idle state not being capable of shortening the time required for connection to the wireless system and not being capable of decreasing the consumption of electric power.